Stars Fly Past Planet, Saving World from Isolation

Stars passing near other stars have long been thought to be culprits in unusual formations of planetary formations, but direct evidence of this idea has not been forthcoming. Now, a team of astronomers believe they have found “smoking gun” evidence of such an event.

A planet orbiting a young binary star designated HD 106906, 300 light years from Earth, shows evidence it may have been saved from being shot into space following a close encounter with a pair of stars. This event took place between two and three million years ago, 12–13 million years after that world formed from a cloud of gas and dust.

An artist’s concept of a giant exoplanet, like HD 106906 b, orbiting its sun, as it may have looked before that world was drawn into the outer reaches of its solar system, far from its parent star. Image credit: NASA/JPL-Caltech

A couple million years in the past, the young planet circled close to the pair of stars around which it orbited, on a path certain to cast it out of the young solar system, into the depths of space as a rogue planet. However, a second pair of binary stars passed by the planetary family, providing a nudge, altering the orbit on which the world traveled, keeping the giant planet in the system.

“One of the mysteries arising from the study of exoplanets is that we see systems where the planets are misaligned, even though they are born in a flat, circular disk. Maybe a cosmic tsunami hit these systems and rearranged everything about them, but we haven’t had proof,” said Paul Kalas, an adjunct professor of astronomy at UC Berkeley.

Some of the more unusual features of our own solar system, including dwarf planets in unusual orbits, and a theoretical ninth planet, could possibly be explained by the passing of a star long ago, astronomers speculate. The search for a similar event in the distant history of our own system is far more difficult, since such an occurrence would have happened over 4.5 billion years ago.

The planet, known as HD 106906 b, has a mass of around 11 Jupiters, and orbits its stellar parent at a an angle inclined 21 degrees to the disk of dust and debris which surrounds the star. The distance between the planet and its stellar companion is also extreme — 738 times the distance between the Earth and the Sun, or 738 astronomical units (AU’s). This is 18 times the distance between Pluto and our own parent star. The young planet is still cooling, showing a surface temperature of 1,500 degrees Celsius (2,700 Fahrenheit).

An image of the area where HD 106906 is found, as well as the two stars which saved a giant world from becoming a rogue planet. Image credit: Paul Kalas/UC Berkeley

“This system is especially fascinating because no model of either planet or star formation fully explains what we see,” said Vanessa Bailey, who led research on the system at the University of Arizona in 2013.

Observations using the Hubble Space Telescope (HST) and the Gemini Telescope in Chile revealed HD 106906 also contains a lopsided belt of comets, providing further evidence of a close encounter with a massive body in the distant past.

Utilizing data from the Gaia space telescope, researchers traced the paths of 461 stars in the same cluster as HD 106906, revealing a close encounter between the two pairs of binary stars two to three million years ago.

“What we have done here is actually find the stars that could have given HD 106906 b the extra gravitational kick, a second kick so that it became long-lived, just like a hypothetical Planet Nine would be in our solar system,” Kalas said.

A mystery since it was found, the size and position of this massive world now has a new origin story. As the young planet orbited its sun, gravitation from the binary stars drove the world into an orbit destined to sent the planet into space. When a pair of binary stars made a close encounter with the system, they drew the world out to a much greater — and safer — distance from its stellar companions, stabilizing its orbit.